Liquid Phase Sintering Behavior Research Articles

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al2O3 and Y2O3 powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al2O3–Y2O3 mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al2O3–Y2O3 mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.

The investigation and regulation of fusion characteristics of coal ash with high sulfur and basic oxides level for the slagging gasifier

The slagging gasifier is one of the best options for coal utilization to achieve carbon neutral due to high carbon conversion and low contaminative gas emission. The slagging gasifier requires proper ash fusibility to ensure smooth slag tapping, but the knowledge concerning fusion behaviors of coal ash with high sulfur and basic oxides level is unclear, which results in the limited utilization of coal which contains high level of sulfur and alkali in ash. Here, the fusion behaviors of coal ash with high sulfur and basic oxide level with/without additives are characterized by ash fusion temperature (AFT) analyzer and thermal mechanical analysis (TMA), and the underlying mechanism is revealed by combination of thermogravimetry (TG), X-ray diffractometer (XRD), and thermodynamic calculation. The results demonstrate that standard ashing procedure impacts the transformation of minerals of coal ash with high sulfur and basic oxide level. The influence of volatiles discharging during standard ash preparation on the shrinkage curve depends on the liquid phase sintering behavior. Besides, the performance of additives connects the ability of additives to volatiles retention. Specifically, accumulation of sulfates by additives leads to the AFTs of Neimeng coal ash increase with increasing additives in the range of 0 to 20%, while the retention of Na-containing minerals by kaolinite gives rise to the dropping AFTs of Wucaiwan coal ash. The study can support the utilization of coal which contains high level of sulfur and alkali in coal ash in the slagging gasifier.

The role of pore evolution during supersolidus liquid phase sintering of prealloyed brass powder

ABSTRACT The aim of this research is to study the pore structure as well as to assess the liquid phase sintering behaviour of Cu-28Zn powder specimens at different green density levels and temperatures. For this purpose, samples were compacted to obtain six different green densities and then sintered at 870°C, 890°C and in part at 930°C for 30 min. The results revealed that the spherical pores which are formed inside the grains can be swept by grain boundaries due to grain growth and join to primary pores so that secondary intragranular pores are eliminated and intergranular pores enlarged at higher temperatures. Also, the pores move upwards to the top of sample due to buoyancy forces. The role of pore structure in distortion is more tangible at higher temperatures (930°C) so that O-shape and X-shape distortions were observed at high and low green density samples, respectively.

Effects of fabrication method on initial powder characteristics and liquid phase sintering behaviour of tungsten

Direct electrochemical production of tungsten involves removal of oxygen in solid state from an important tungsten compound, calcium tungstate, in molten salts. A detailed experimental study was performed to characterize the liquid phase sintering behaviour of the tungsten powder (WE), which was produced by the direct electrochemical reduction technique. All of the tungsten heavy alloy (WHA) compositions covered in this study, contained four parts nickel, one part cobalt and over 90 wt% tungsten. The pellets were sintered at 1500 and 1550 °C under continuous flow of an equimolar hydrogen-argon gas mixture. A commercially available tungsten powder (WC) was also used to prepare the same tungsten heavy alloy compositions as those prepared by the electrochemically produced tungsten powder to act as a control group. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) and hardness measurements were employed for characterization. The analyses provided that; the tungsten powder as obtained from the direct electrochemical reduction process contained some unreduced CaWO4 particles and they limited the grain growth and mass transport in the liquid phase. Although calculated relative densities were a bit lower than desired, the tungsten heavy alloys produced by the electrochemical reduction technique was promising for use when average grain size, dissolved tungsten contents and general/matrix hardness values are considered.

One step sintering of homogenized bauxite raw material and kinetic study

A one-step sintering process of bauxite raw material from direct mining was completed, and the kinetics of this process was analyzed thoroughly. The results show that the sintering kinetics of bauxite raw material exhibits the liquid-phase sintering behavior. A small portion of impurities existed in the raw material act as a liquid phase. After X-ray diffraction analyses, scanning electron microscopy observations, and kinetics calculations, sintering temperature and heating duration were determined as the two major factors contributing to the sintering process and densification of bauxite ore. An elevated heating temperature and longer duration favor the densification process. The major obstacle for the densification of bauxite material is attributed to the formation of the enclosed blowhole during liquid-phase sintering.

Effect Of Compaction Pressure And Sintering Temperature On The Liquid Phase Sintering Behavior Of Al-Cu-Zn Alloy

Abstract The liquid phase sintering characteristics of Al-Cu-Zn alloy were investigated with respect to various powder metallurgy processing conditions. Powders of each alloying elements were blended to form Al-6Cu-5Zn composition and compacted with pressures of 200, 400, and 600 MPa. The sintering process was performed at various temperatures of 410, 560, and 615°C in N2 gas atmosphere. Density and micro-Vickers hardness measurements were conducted at different processing stages, and transverse rupture strength of sintered materials was examined for each condition, respectively. The microstructure was characterized using optical microscope and scanning electron microscopy. The effect of Zn addition on the liquid phase sintering behavior during P/M process of the Al-Cu-Zn alloy was also discussed in detail.

Effect of sintering conditions on microstructure and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics

The influence of sintering temperature and dwell time on the microstructure formation and dielectric properties of CaCu3Ti4O12 ceramics was investigated. For sintering temperatures of 1050 and 1100 °C significant differences in the CaCu3Ti4O12 ceramic microstructure and the segregation of a CuOx-rich phase towards the grain boundary (GB) areas were observed with increasing dwell time. In addition to the formation of a semiconducting bulk and insulating grain boundary phase the segregated CuOx forms an intergranular phase, and the effects of this phase on the dielectric properties are rather intriguing. At sintering temperature below 1050 °C only small amounts of CuOx segregate, whereas sintering above 1050 °C (e.g., 1100 °C) leads to increased evaporation of the CuOx. Therefore, the effects of the CuOx-rich intergranular phase upon the dielectric properties are felt strongest in samples sintered at 1050 °C. Such effects are discussed in terms of microstructural variations due to liquid phase sintering behavior facilitated by the TiO2-CuOx-eutectic, which appears to be melted at high sintering temperature prior to evaporation of CuOx at prolonged dwell times at the highest sintering temperatures (1100 °C).

Influence of lithium oxide additives on densification and ionic conductivity of garnet-type Li6.75La3Zr1.75Ta0.25O12 solid electrolytes

Abstract The lithium garnet ceramic has been considered as one of promising solid-state electrolytes for secondary lithium batteries. However, improvement of its ionic conductivity is hindered by the high porosity related to the severe volatilization of lithium components during sintering. In order to find the solution for this problem, Li2O additives in concentration of 2–8 wt.% were introduced into the cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) at the solid-state reaction processes. It is found that the Li2O additives lead to formation of glassy-like phases at the grain boundaries related to the liquid-phase sintering behavior, which eliminates the residual pores therein and increases the relative density from 91.5% to 97.3%. Measurement of conductivity indicates that 6 wt.% is the optimum concentration of Li2O which leads to an ionic conductivity of 6.4 × 10− 4 S cm− 1 at room temperature. This value is approximately 3 times larger than that of the ceramic without the additive.

Liquid phase sintering behavior and improvement of giant dielectric properties by modifying microstructure and electrical response at grain boundaries of CaCu3Ti4−xMoxO12 ceramics

Abstract The dielectric properties and high-temperature electrical response at grain boundaries (GBs) of CaCu 3 Ti 4 O 12 ceramics were improved by doping with Mo 6+ . The mean grain size of CaCu 3 Ti 4− x Mo x O 12 ceramics considerably increased with increasing Mo 6+ , resulting from the existence of a MoO 3 -based liquid phase(s) during the sintering process. Enlargement of grain size was found to be accompanied by a segregation of a Cu-rich phase at GBs. The dielectric permittivity increased significantly with increasing Mo 6+ , whereas, the loss tangent at room temperature increased slightly. Interestingly, substitution of 0.1 at.% Mo 6+ suppress the high-temperature loss tangent to less than 1.0 over the temperature range of −70 to 200 °C. Substitution of Mo 6+ into Ti 4+ sites could be electrically compensated by filling oxygen vacancies at the GBs. This led to an increase in the conduction activation energy at the GB region, resulting in suppression of conduction through the GBs and reduction in loss tangent at high temperatures.

Sintering Characteristics of Micro-Zn Paste Containing Sn-3.0Ag-0.5Cu Nanoparticles

The transient liquid-phase sintering behavior of micro-Zn/nano-Sn-3.0Ag-0.5Cu (SAC) pastes was examined at a temperature of 190 °C as a function of the volume of ultrafine (~12.4 nm) SAC nanoparticles present. SAC nanoparticles have lower melting point drop than the bulk SAC particles. Although successful linkage at the interface between all Zn particles was not accomplished in all sintered samples, the number of linkages increased marginally with a decrease in the SAC content. As a result, the electrical resistivity of the sintered samples decreased with the decrease in the SAC content; however, the resistivities were still very high in all samples. Microstructural observations indicated that the observed results were mainly due to the short lifespan of the liquid phase caused by the coarsening of SAC nanoparticles during heating.

Liquid Phase Sintering Behavior of Amorphous Nano-Sized Silicon Nitride Powders

The amorphous nano-sized silicon nitride powders were sintered by liquid phase sintering method. Si3N4-Si2N2O composites were in-situ fabricated. The Si2N2O phase was generated by an in-situ reaction 2Si3N4(s)+1.5O2(g)=3Si2N2O(s)+N2(g). The content of Si2N2O phase up to 60% was obtained at a sintering temperature of 1650°C and reduced when the sintering temperature increased or decreased, which indicates that the reaction is reversible. The mass loss, relative density and average grain size increase with increasing of sintering temperature. The average grain size is less than 500nm when the sintering temperature is below 1700°C. During the sintering procedure, there is a complex crystallization and phase transition: amorphous Si3N4 → equiaxial α-Si3N4→ equiaxial β-Si3N4 → rod-likeSi2N2O → needle-like β-Si3N4. Small round-shaped β-Si3N4 particles are entrapped in the Si2N2O grains and a high density of staking faults are situated in the middle of Si2N2O grains at a sintering temperature of 1650°C.

7xxx계 Al 혼합분말의 승온속도에 따른 소결거동

7xxx series Al alloy has the most attractive properties including its excellent high specific strength, stress corrosion cracking and corrosion-resistance. However, in case of the Al-Zn system, the liquid phase has a transient aspect because of the high solid solubility of Zn in Al. Therefore, transient liquid phase sintering behavior was observed during the sintering process. And the amount of liquid and its duration were influenced by the process variables including heating rate and final sintering temperature. At high heating rates(<TEX>$100^{\circ}C/min$</TEX>), the liquid fraction increased during sintering because diffusion was minimized and therefore local saturation could easily occur. The sintered density increased with increasing heating rate.

Effects of Fe-TiB2 Addition as Binder Phase on the Liquid Phase Sintering Behaviour of Tungsten Carbide Cermets

Effects of Fe-TiB2 Addition as Binder Phase on the Liquid Phase Sintering Behaviour of Tungsten Carbide Cermets

フェライト系焼結ステンレス鋼の液相焼結挙動と耐食性に及ぼすボロンおよびシリコン添加の影響

The effect of boron (B) and silicon (Si) additions on the liquid-phase sintering behavior and corrosion resistance of P/M ferrite type stainless steel (SUS430L) was investigated. As a result, the addition of B improved the sintering of specimens, and the relative sintered density of 0.2mass%B and 0.2mass%B+2mass%Si added steel sintered at 1573 K were more than 97%. The high tensile strength was obtained on 0.2mass%B+2mass%Si added specimens sintered at 1473, 1523 and 1573 K, and the maximum strength was 508 MPa obtained at 1573 K. The reason was considered due to the high density of the matrix obtained by the eutectic reaction between the matrix and borides and the solid-solution hardening of matrix by Si element. On the other hand, the ductility was not improved by the addition of B and Si. This results may be due to grain coarsening and borides precipitated along the grain boundary. The specimen with 0.2mass%B and 0.2m, -ss%B+2mass%Si sintered at 1573 K were found to have an excellent corrosion resistance in a pitting corrosion test of a solution of 6 % FeCl3 (N/20 HC1). Especially, the corrosion rate of the 0.2mass%B + 2mass%Si added steel sintered at 1573 K is smaller by one sixth compared of the non-added steel. However, in the case of steels sintered at lower than 1523 K, the corrosion resistance of 0.2mass%B and 0.2mass%B+2mass%Si added steels was similar to that of the non-added steel. This tendency was probably due to a low sintered density in similar to the non-added P/M steel.

A study on the improvement of the sintered density of W-Ni-Mn heavy alloy

Liquid phase sintering behavior of 90W-6Ni-4Mn heavy alloy has been studied. The present work takes into account the thermodynamic oxidation/reduction reactions of the constituent elements W, Ni, and Mn. The sintering cycle consists of heating under high purity nitrogen gas, holding at reduction temperatures after the atmosphere is changed to dry hydrogen, and sintering at 1260 °C for 1 hour. As the reduction temperature increases from 1050 °C to 1200 °C, the relative sintered density increases from 92 pct, reaching 100 pct at temperatures above 1150 °C. The relative density increases with increasing holding time at the reduction temperature and remains unchanged with heating rate. The sintered microstructure has been analyzed by a scanning electron microscope and energy dispersive X-ray spectroscopy. The relative density was compared with those obtained from other investigators. It is found that the formation of manganese oxide due to reducing reactions of W and Ni powders should be avoided in order to obtain a W-Ni-Mn heavy alloy without pores.

Effect of particle size on the sintering of Li2O-Al2O3-4SiO2-borosilicate glass composites

The effect of spodumene particle-size on the sintering of spodumene-borosilicate glass composites was investigated. The results can be explained by a particle-size dependence of the extent of reaction between spodumene and glass. For samples fabricated with fine spodumene particles (1.5–3 μm), a reactive liquid-phase sintering behaviour was observed. Smaller particle size resulted in a higher sintered density and coarser microstructure. For samples fabricated with coarse spodumene particles (3–7 μm), a non-reactive liquid-phase sintering behaviour was observed when a high-viscosity glass, a lower initial glass content, and a lower sintering temperature were used. A larger particle size resulted in a higher sintered density and coarser microstructure. If a low sintering temperature or a coarse (e.g., 7 μm) particle size were used, the dissolution of the glass into spodumene was reduced which resulted in an increased amount of glass crystallization.

Comparisons in the Liquid Phase Sintering Behaviour of Different High Speed Steels with Copper Phosphorus Addition

Comparisons in the Liquid Phase Sintering Behaviour of Different High Speed Steels with Copper Phosphorus Addition

Sintering behavior of biporous molybdenum-copper pseudoalloys

Solid- and liquid-phase sintering experiments on a biporous molybdenum-copper material have been found to be accompanied by an increase in pore size at the macroporosity level, which is a manifestation of the law of zonal separation. It is shown that in the sintering of a biporous material processes taking place at the microporosity level determine the structural characteristics of the material at the macroporosity level. Some characteristics of the solid- and liquid-phase sintering behavior of the material investigated have been established.

Sintering Studies on the Talc-Kaolin and Forsterite-Kaolin Systems

The systems, talc-kaolin and forsterite-kaolin show a typical liquid phase sintering behaviour. Densification and microstructure studies on the systems talc-kaolin and forsterite-kaolin show that the rate of densification is controlled by diffusion through liquid film between particles. Experimental measurements are in reasonable agreement with theoretical predictions.